Nowadays, frequency reuse-1 is preferred in the terms of frequency efficiency. However, in cellular communication systems, the choice in favor of reuse-1 results in interference from the neighbor bases stations which may not be negligible. In order to ensure reliable communication, this interference should be appropriately handled. However, the treatments of the interference tend to require more complicated development in the whole system, such as intelligent beamforming antenna, or network coordination.
The dominant co-channel interference appears often at the cell edges where the handoff is about to happen. Before the handoff, the targeted base station is actually the dominant co-channel interference. And, in most communication systems, the handoff operation is often triggered when the SINR (Signal to Interference-plus-Noise Ratio) value is below a certain threshold. This SINR threshold may be set to a very low level in order to avoid the “ping-pong” effect; however, this causes the mobile station to perform in a difficult situation with strong interference.
For example, in FIG. 1, mobile station MS receives the signals from serving base station SBS with power C, and it also receives the interference from the dominant interfering base station IBS with power Id. Generally, at the cell edges, such as at the point A, the serving signal power level C is comparable with the dominant interference level Id. But the system still “observes” the targeted interfering base station IBS behavior before switching the connection to it. For instance, the handoff is launched when the mobile station MS is in the position B. From the curves in FIG. 1, we can deduce that while the mobile station BS is moving from point A to point B, the communication quality with the serving base station SBS gets worse since the interference level Id becomes stronger than the desired power level C. Accordingly, the connection can be lost before the handoff operation is successfully completed.
It is worth noting that the increased interference phenomenon can be used as a trigger for handoff. Furthermore, with appropriate interference exploitation, the handoff threshold can be set to even a lower level in order to avoid “ping-pong” effect.
This interference issue could be highly critical in practical communication systems and proper handling of the interference results in a considerable performance improvement. For example, in WiMAX systems, the first PUSC (partial usage of subcarriers—to use a predetermined number of subcarries instead of using all the subcarries) zone contains FCH (frame control header) which contains the information of DL-MAP and UL-MAP), DL-MAP (allocation information of DL subframe) and UL-MAP (allocation information of UL subframe) information, and this zone gives the allocation information of the data zone (zone following the 1st PUSC zone) from the corresponding base station. The data part is less affected by the interference from interfering base stations because, its performance loss can be compensated by some retransmission mechanisms. However, particularly in reuse 1 deployments, different base stations transmit their signals in the first PUSC zone by using the same frequency resources. The FCH/DL-MAP is transmitted in the first slot array and interfered by other base stations.
Therefore, without the reliable decoding of FCH/DL-MAP part, the users may suffer from the co-channel interference and this results in a poor connection quality. As discussed above, this is particularly important during handoff operation. When the received power from the interfering BS becomes stronger (e.g., moving from point A to point B in FIG. 1), the FCH/DL-MAP part of the serving base station becomes more and more vulnerable. In such a case, while mobile station is negotiating to switch to the powerful interfering base station, the connection can be lost.